It is known that Stirling engines are heat engines of the external heating, piston type, in which the working gas, which is helium or hydrogen, circulates alternately from one cylinder to another, passing through a tubular heat exchanger in which it is heated, a porous metallic matrix known as a regenerator, in which it receives or gives up heat, and another tubular heat exchanger in which it is cooled. The two heat exchangers also enable it to retain its temperature when it is compressed or expanded in the cylinders.
A two-cylinder engine of this type is illustrated diagrammatically in FIG. 1. A kinematic connection comprising connecting rods and a crankshaft is provided between the two pistons in such a manner that the movement of the piston of the "cold" cylinder B is offset 90.degree. C. in relation to that of the "hot" cylinder A. When the crankshaft F turns about one sixth of a turn in the direction indicated by the arrow, starting from the position shown in FIG. 1, the piston of cylinder A moves very little, while the piston of cylinder B compresses the gas which is kept cold by the cooler C. When the next sixth of a revolution is made, the gas is transferred from cylinder B to cylinder A while being heated by the regenerator D and the heater E. During the next sixth of a revolution, the piston of cylinder B moves very little, while the piston of cylinder A allows the gas kept hot by the heater E to expand. In the next half-revolution the gas is transferred from cylinder A to cylinder B while being cooled by the regenerator D and the cooler C, and a new cycle starts. In the course of this complete cycle, as in any heat engine, heat is supplied at high temperature (in the heater), heat is given up at low temperature (in the cooler), and work is done (on the crankshaft). Heating in the heater E is generally effected by passing through it combustion gases from various materials, and cooling is effected in the cooler C by means of a current of cold water.
A known arrangement of this kind can be modified as shown in FIG. 2. The variable cold volume is disposed not above, but below the piston of cylinder B, on the kinematic connection side. The operating principle of the engine is not changed, provided that the movement of the piston of cylinder B is 90.degree. ahead in phase in relation to that of cylinder A, as is shown in this Figure.
The diagram given in FIG. 2 makes it possible to conceive the connection in series of a plurality of engine units of the kind shown in FIG. 2, as is suggested by the pipe outlets shown on cylinders A and B. A double-acting piston engine is then obtained in which each of the pistons works on both its faces, and the power of which is substantially doubled for a given cylinder capacity. Furthermore, the devices sealing the engine in relation to the outside are situated under the cold variable volumes of the cylinders, so that their construction is less difficult. The various engine units, which are generally from three to six in number, preferably form a closed chain in a so-called RINIA arrangement; see, for example, G. Walker, "Stirling Engine Machines", U. of Bath 1971. This principle has been adopted for almost all existing Stirling engines.
For good thermodynamic functioning it is necessary in practice that the hot ducts between A and E in FIG. 2 and the cold ducts between B and C, constituting dead volumes and causing pressure drops, should be shortened as much as possible; in order to reduce deformation, play and friction, it is also necessary that the kinematic connection between the various piston rods should simplified as far as possible. Finally, when heating is effected by combustion gases, it is necessary for the tubes of the heater E to be grouped to form a single heating head permitting uniform distribution of the heat.
A very compact engine complying with the conditions set forth above is obtained by disposing four cylinders in a square. The piston rods may be parallel and may operate an inclined plate (swash plate or wobble plate) or a double crankshaft (H type engine); they may also be inclined in V form and drive a single crankshaft (double V type engine). These various arrangements are those most generally adopted in present Stirling engines.
As a setoff to compactness, these Stirling engines lose the modular character which it would have been possible to obtain by connecting a plurality of engine units in series in accordance with the diagram shown in FIG. 2. They are composed of a single engine assembly in which only small components, such as regenerators, coolers or pistons, are manufactured as sets of identical parts.